Vocus PTR-TOF Participates in Intensive Ambient Air Analysis Intercomparison

Together with our partner Aerodyne Research, we recently deployed a Vocus PTR-TOF 2R to the CESAR observatory in the Netherlands for participation in an ACTRIS intensive PTR-MS intercomparison.  ACTRIS (Aerosols, Clouds and Trace gases Research Infrastructure) is an initiative to coordinate the efforts of European researchers in order to produce and share high-quality observations of aerosols, clouds and trace gases.  This 2-week campaign aimed to evaluate and compare best practices for PTR-MS calibration and background subtraction so as to improve measurement accuracy and precision of key atmospheric VOCs.  The Vocus PTR-TOF 2R ran alongside 11 other PTR-MS at the CESAR observatory, 8 of which also used TOFWERK TOF mass analyzers.

The ACTRIS campaign took place at the CESAR observatory in the Netherlands. 12 PTR-MS measured VOCs in ambient air for two weeks, sampling from a shared 10-meter sampling line.

The VOCUS 2R made continuous 2-Hz measurements (2 mass spectra/second) for the entire two weeks, sampling ambient air from a shared 10-meter sampling line. The instrument ran completely autonomously, including hourly automatic calibrations with a certified gas standard, and 1-minute automatic background measurements every 30 minutes. An example of a 2-Hz xylene time series is shown here. The inset demonstrates the high frequency response and precision afforded by the high sensitivity of the Vocus.

Example time series of ambient xylene concentrations recorded with the PTR-TOF 2R at 2 Hz. The inset demonstrates the precision and fast response during a period when concentrations were between 150 and 350 pptv.

The combination of high sensitivity and high mass resolving power (~12,000 as tuned during this campaign) enables unambiguous spectral identification at unprecedented speed. The example below shows high resolution separation and identification at 105 Th. The data clearly reveal 5 distinct ions which would be difficult to assign and quantify without this level of resolving power.

The Vocus 2R is able to resolve many ions at each nominal mass. As an example, data at mass/charge 105 Th show 5 distinct ions. Unique time series are reported for each of these ions.

The measurements and detailed instrument intercomparisons made during the ACTRIS campaign will be the basis of forthcoming publications.

Tofwerk and Aerodyne kindly thank Prof. Rupert Holzinger at Utrecht University for organizing this intercomparison and inviting us to participate.